The nuclear magnetic resonance spectra of porphyrins. Part X. Carbon-13 nuclear magnetic resonance spectra of some meso-tetraarylporphyrins and their metal chelates

Abstract

The ¹³C n.m.r. spectra of meso-tetraphenylporphyrin (TPP), the zinc(II), cadmium(II), bis[mercury(II) acetate], and various thallium(III) derivatives, and some related analogues are reported and assigned. In meso-tetra-(o-tolyl)-porphyrin and meso-tetra-(1-naphthyl)porphyrin only one species was observed. The introduction of zinc(II), cadmium(II), or bis[mercury(II) acetate] into the TPP nucleus does not significantly change the ¹³C shifts, but the thallium(II) derivative shows both extensive TI–¹³C couplings and non-equivalence of o-phenyl carbon atoms, the latter being due to the exoplanar position of the thallium atom. Specific TI–¹³C couplings to the o-phenyl carbon atoms are also observed. The phenomenon of slow protonation (TPP ⇄ TPPH₂ ²⁺) was identified and examined by using ¹³C and ¹H n.m.r. spectroscopy, and the ¹³C shifts and assignments for the dication are reported. The large protonation shifts of TPP and high ΔG value for this process (ca. 16·3 kcal mol^–1) are consistent with the further buckling of the macrocycle on protonation, due to decrease in the angle between the macrocycle and the phenyl rings. In contrast, the protonation shifts of meso-tetra-(o-tolyl)porphyrin are ‘normal’ owing to the additional steric requirements of the o-methyl groups. A hitherto unrecorded effect is the shift of the tetramethylsilane reference in trifluoroacetic acid compared with deuteriochloroform of 1·5 p.p.m. upfield with respect to other reference compounds. Identification of the coalescence temperature for the NH tautomerism in both the ¹³C and ¹H spectra allows the kinetic parameters for this process to be determined (ΔG₃₀₃ca. 12·3 kcal mol^–1, ΔH ca. 9·2 kcal mol^–1, and ΔS ca.–10 cal K^–1 mol^–1). These, and the isotope effect (K_NH/K_ND) of ca. 12 (measured at one temperature to remove entropy of activation effects) are consistent with a two-step (rather than a concerted) process.